A nutritionist reveals the healthiest things to eat on a plane

• INSIDER spoke to nutritionist Karen Ansel to find out the healthiest food to eat while flying. • Pack a peanut butter or turkey sandwich instead of ordering off an in-flight menu. If you have to order something on the plane, opt for a healthy sandwich with lots of vegetables.• Finally, drink plenty of water, and avoid alcohol.

She told INSIDER that the healthiest meal you can eat on a flight is "a turkey or peanut butter sandwich on whole wheat bread, plus an apple." It's easy to make, compact enough to fit inside your carry-on, and packed with protein, plus "it will keep you full for hours," she said.

If you can't prepare a meal ahead of time, you can also make a quick version on the go. The nutritionist advises "put[ting] together a healthier meal piecemeal by buying a banana, a container of Greek yogurt, and a grain and nut based snack bar like Kashi Chewy Nut Butter bars."

Failing that, choose something sensible from the airline's in-flight menu. "If you absolutely have to buy something [on the plane], make it a turkey sandwich on whole wheat and pile on the veggies like lettuce, tomato, cucumber and avocado," Ansel said.

She also cautions flyers to watch out for added dressings and condiments, which often pack extra calories. "Make sure to skip the mayo and opt for mustard or a little olive oil and vinegar instead."

As well as eating well on a flight, Ansel said drinking plenty of water is a must.

"When you're flying dehydration is a real problem," she said. "Nobody wants to use an airplane bathroom if they don't have to, so it's natural to try to drink as little as possible. Unfortunately, this can really mess with your hydration, because airplane air is very dry, so you're losing water all throughout your flight."

Ansel recommends drinking something at least "every couple of hours," even if it's not water. "Water is ideal, but other than alcohol [which dehydrates] just about any other fluid is fine," she said.

Surprisingly, Ansel isn't against drinking coffee at altitude. "If you like coffee, go for it," she said. "Contrary to what we've been told, caffeinated drinks are not dehydrating. One study even found that coffee can hydrate you almost as well as water does."